AN X Y DNA SEGMENT FROM AN EARLY-STAGE OF SEX-CHROMOSOME DIFFERENTIATION IN THE FLY MEGASELIA-SCALARIS/

The sex chromosomes of the Megaselia scalaris wild-type strain Wien ar e homomorphic. We studied a roughly 1.8 kb X/Y DNA segment of this str ain. It includes, at one end, the first part of a coding sequence for a protein of the vespid antigen 5 family. Molecular differentiation be tween the X and Y chromosomes has commenced, but homology, even of sho rt DNA stretches, is still assessable beyond doubt. The most conspicuo us differences between the X and the homologous Y segment were inserti ons/deletions in the noncoding region: among them, deletions, a duplic ation, and an insertion of a mobile element. These structural changes grossly disrupted homology. In comparison, base substitutions, though mon numerous, contributed little to the differentiation of the X/Y DNA segment.